In a prior-art buckle folding machine (W. German patent Specification No. 27,57,182), a material guide is arranged each in front of and behind a pair of folding rollers, if viewed in the direction of transportation of the material to be folded. To interrupt the feed of the material to be folded in a preselectable manner, corresponding to the desired folding length, and to bend over the material to be folded to form a fold, feed limiters in the form of clamping members of pressure rollers are provided. These feed limiters act on the material to be folded by friction and stop the section of material being fed and/or drive it in the opposite direction. The pressure rollers press the section of material being fed against the folding roller located in the vicinity of the inlet of the material guide. The feed limiters are actuated by sensors which are arranged in the feed paths in front of each pair of folding rollers and sense the leading edge of the material being fed. The feed limiters are actuated by electromagnets, which are controlled in agreement with the desired folding length by electronic pulse generators synchronized with the folding rollers. Stationary material sensors, which sense the sections of material being fed, are mounted in the zone of the buckling space. The pulses of the pulse generator are sent as counting pulses to a presettable coincidence circuit as a function of the state or state of the material sensors, wherein the coincidence circuit ensures actuation of the feed limiters and also cancellation of the counting pulses in the coincidence circuit in the case of coincidence of the pulse count with preset number.
In this as well as other, prior-art buckle folding machines (W. German patent specification No. 673,176 and W. German Patent Specification No. 519,140), the feed limiters which are provided in the feed paths instead of the adjustable paper stops, which are otherwise commonly used in buckle folding machines, are braking or stopping means whose function is based on clamping or friction, and which block the sections of material being fed and prevent them from being fed further by pressing the material to be folded by frictional engagement either against a non-rotating surface or against the rotating shell surface of a folding roller on being actuated electromagnetically, in order to bring about folding and the feeding of the material to be folded to the nearest folding zone.
Since the mode of operation of these prior-art feed limiters is based on the generation of frictional engagement between the material to be folded and a braking member or a drive member rotating in the opposite direction, the result of the operation is unsatisfactory in terms of the folding length tolerances, especially at high working speeds.